1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device and a method for fabricating the same. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for providing maximum liquid crystal efficiency from all areas of a pixel.
2. Discussion of the Related Art
Recently, as the capacity and performance of active liquid crystal display devices are developing and a fast rate, flat panel televisions, portable computers, monitors, and so on are being used extensively. Among the active liquid crystal display devices, twisted nematic (TN) liquid crystal display devices are most generally used. In the twisted nematic liquid crystal display device, an electrode is formed on each of two substrates. Herein, the electrodes are aligned so that liquid crystal directors are twisted to 90°. Then, voltage is applied to the electrodes so as to operate the liquid crystal director.
The twisted nematic liquid crystal display device is highly acknowledged for its excellent contrast and color representation. However, the twisted nematic liquid crystal display device is disadvantageous in that it has a very narrow viewing angle.
In order to overcome the problem of a narrow viewing angle of the twisted nematic liquid crystal display device, an in-plane switching (IPS) mode liquid crystal display device is used. In the IPS mode liquid crystal display device, two electrodes are formed on a single substrate so that a transverse electric field formed between the two electrodes can control the liquid crystal director.
Thereafter, a fringe field switching (FFS) mode liquid crystal display device has been proposed in order to enhance the low aperture ratio and transmissivity of the IPS mode liquid crystal display device. In the FFS mode liquid crystal display device, a counter electrode and a pixel electrode are formed of a transparent conductive material. And, the counter electrode and the pixel electrode are formed to be spaced apart at a close distance. A fringe field formed between the counter electrode and the pixel electrode operates the liquid crystal molecules.
Hereinafter, the FFS mode liquid crystal display device and the IPS mode liquid crystal display device of a thin film transistor (TFT) array panel will now be described in detail.
FIG. 1 illustrates a plane view of a related art IPS mode liquid crystal display device. And, FIG. 2 illustrates a cross-sectional view taken along line I-I′ of FIG. 3. FIG. 3 illustrates a plane view of a related art FFS mode liquid crystal display device. And, FIG. 4 illustrates a cross-sectional view taken along line II-II′ of FIG. 3.
Referring to FIG. 1 and FIG. 2, the IPS mode liquid crystal display device includes a gate line 12 and a data line 15, a thin film transistor, and a common electrode 24 and a pixel electrode 17. The gate line 12 and the data line 15 perpendicularly cross one another so as to define a unit pixel. The thin film transistor is formed at a crossing point (or intersection) between the gate line 12 and the data line 15. Then, the common electrode 24 and the pixel electrode 17 are alternately formed so as to be parallel to one another. Herein, a transverse electric field is generated between the common electrode 24 and the pixel electrode 17. At this point, the common electrode 24 and a common line 25 are formed as a single body, the common line 25 being parallel to the gate line 12. The common electrode 24 receives voltage from outside of the active area.
As described above, the common electrode 24 and the pixel electrode 17 are formed on the same substrate. Then, the voltage is applied between the two electrodes so as to generate a transverse electric field, which is parallel to the substrate. Thus, the transverse electric field can rotate the liquid crystal molecules, while the liquid crystal molecules remain parallel to the substrate. The IPS mode liquid crystal display device further includes an orientation layer, which is formed on the inner surface of each substrate. The orientation layer of the upper substrate and the orientation layer of the lower substrate are rubbed so that each orientation direction is parallel to one another.
Hereinafter, the method for fabricating the IPS mode liquid crystal display device will now be described in detail.
First of all, a gate material is deposited on a substrate 11 and patterned. Then, a plurality of gate lines 12, gate electrodes 12a, common lines 25, and common electrodes 24 are formed thereon. Subsequently, an insulating material is deposited on the entire surface including the gate lines 12 so as to form a gate insulating layer 13. Then, a layer of amorphous silicon (a-Si:H) is deposited on the gate insulating layer 13 at a high temperature and patterned, thereby forming a semiconductor layer 14 on the gate insulating layer 13 formed over the gate electrode 12a. 
Thereafter, a low-resistance metal layer is deposited on the semiconductor layer 14, thereby forming a plurality of data lines 15, and source/drain electrodes 15a/15b. Then, a layer of inorganic insulating material or organic insulating material is deposited on the entire surface including the data lines 15, so as to form a protective layer 16. Afterwards, a portion of the protective layer 16 is removed so as to form a contact hole, which exposes the drain electrode 15b. 
Finally, a layer of transparent conductive material, such as indium tin oxide (ITO) or indium zinc oxide (IZO), is deposited on the protective layer 16 and patterned, thereby forming the pixel electrode 17. The pixel electrode 17, which is parallel to the common electrode 24, passes through the contact hole and contacts the drain electrode 15b. Subsequently, an orientation layer (not shown) is formed on the pixel electrode 17, which is then oriented to a desired direction, thereby completing the IPS mode liquid crystal display device. Generally, the IPS mode liquid crystal display device maintains an orientation angle of 180° between the orientation layer of the upper substrate and the orientation layer of the lower substrate, regardless of whether the orientation is directed left-to-right or right-to-left.
However, the IPS mode liquid crystal display device shows a transmissivity dispersion as shown in FIG. 2. More specifically, in area {circle around (1)}, which is between the pixel electrode 17 and the common electrode 24, the liquid crystals 30 are tilted due to the transverse electric field, thereby showing high transmissivity. In area {circle around (2)}, which is above the pixel electrode 17 and the common electrode 24, a portion of the liquid crystals 30 is moved in a vertical direction, thereby causing a decrease in transmissivity and brightness.
On the other hand, referring to FIG. 3 and FIG. 4, the FFS mode liquid crystal display device includes a gate line 512 and a common line 525, a thin film transistor, and a plate-type counter electrode 524 and pixel electrode 517. The gate line 512 and common line 525 are each formed of opaque metal perpendicularly crossing one another, so as to define a pixel. The thin film transistor switches the voltage on/off at each intersection between the gate line 512 and common line 525. The plate-type counter electrode 524 and pixel electrode 517 are formed of transparent metal. The plate-type counter electrode 524 and pixel electrode 517 are insulated by an insulating layer and overlap one another within each pixel. At this point, the counter electrode 524 and the common line 525 contact one another. The common line 525 is fixed to be parallel to the gate line 512, thereby dividing the pixel to an upper portion and a lower portion.
More specifically, the pixel electrode 517 is formed of a transparent plate-type metal. A plurality of slits 560 is formed to be vertically symmetrical to one another along the axis of the common line 525. And, a fringe field is generated between the counter electrode 524 and the pixel electrode 517. At this point, a Vcom signal is transmitted to the counter electrode 524, and a pixel voltage, which passes through the thin film transistor, is transmitted to the pixel electrode 517.
Each of the slits 560 has a width within the range of 2 to 6 micrometers (μm). And, the liquid crystals are driven by the fringe field generated between the pixel electrode 517 and the counter electrode 524. In other words, when voltage is not applied, the initially oriented liquid crystals rotate due to the fringe field, thereby allowing light to pass through. The FFS mode liquid crystal display device further includes an orientation layer, which is formed on the inner surface of each of the upper and lower substrates. The orientation direction between the orientation layer of the upper substrate and the orientation layer of the lower substrate maintains 90°, which is identical to the general TN mode liquid crystal display device.
Hereinafter, the method for fabricating the FFS liquid crystal display device will now be described in detail. First of all, a transparent conductive material, such as ITO, is deposited on the substrate 511 and patterned, so as to form a counter electrode 524. A gate material is deposited on the counter electrode 524 and patterned, thereby forming the gate line 512 and the common line 525. A portion of the gate line 512 becomes the gate electrode. And, the gate line 512 and the common line 525 are parallel to one another.
Thereafter, an insulating material is deposited on the entire surface of the substrate having the counter electrode 524, the gate line 512, and the common line 525 formed thereon, thereby forming a gate insulating layer 513. Then, a semiconductor layer 514 is formed on a portion of the gate insulating layer 513 above the gate electrode. Subsequently, a data material is deposited on the entire surface of the structure including the semiconductor layer 514 and patterned, so as to form a data line 515, and source/drain electrodes 515a and 515b. Then, an insulating material is deposited on the entire surface of the structure including the data line 515, thereby forming a protective layer 516.
Finally, a transparent conductive material, such as ITO, is deposited on the protective layer 516 and patterned, so as to form the pixel electrode 517 having a plurality of slits 560. Then, an orientation layer (not shown) is formed, which is then oriented to a desired direction, thereby completing the FFS mode liquid crystal display device. Generally, the FFS mode liquid crystal display device maintains an orientation angle of 90° between the orientation layer of the upper substrate and the orientation layer of the lower substrate, regardless of the orientation direction.
However, the FFS mode liquid crystal display device shows a transmissivity dispersion as shown in FIG. 4. More specifically, in area {circle around (1)}, which is above the pixel electrode 517, the liquid crystals 530 are tilted, thereby showing high transmissivity. In area {circle around (2)}, which is above the edge portion of the pixel electrodes 517 and above the slits formed between the pixel electrodes 517, a portion of the liquid crystals 530 is moved in a vertical direction, thereby causing a decrease in transmissivity and luminance.
As described above, the related art liquid crystal display device has the following disadvantages.
The IPS mode liquid crystal display device, which is twisted by 180° and rubbed, may yield a maximum liquid crystal efficiency (0.8) in an area between the pixel electrode and the common electrode. However, the liquid crystal efficiency decreases noticeably (less than 0.4) in an area above the pixel electrode and the common electrode.
Conversely, the FFS mode liquid crystal display device, which is twisted by 90° as in the twisted nematic quid crystal display device and rubbed, yields a high liquid crystal efficiency (0.6 or more) above the electrodes. However, the liquid crystal efficiency decreases either in the edge portion of the pixel area or the slit area between the pixel electrodes.